One disclosure of such a prosthesis is to be found in WO-A-02/15820 of the present applicant. The WO document discloses a self-expanding stent of nickel titanium shape memory alloy that carries a ring of spoons of tantalum at each end of the tube of the stent, these tantalum spoons serving as radiopaque markers so that the stent prosthesis can be located by radiographic techniques, when inside the body of a patient.
It is customary to polish a prosthesis during manufacture, prior to implantation in the body, for the obvious reason that, at the moment of implantation, there should be no surfaces in a condition of anything less than full integrity. Polishing is conventionally accomplished by an electro-polishing procedure and, in electro-polishing, the rate of removal of solid material from the surface being polished will vary, according to the chemical composition of the surface and the chemical composition of the fluid medium in contact with that surface. For example, in the case of a NITINOL nickel titanium memory metal stent with tantalum radiopaque markers, electropolishing can remove NITINOL four times as fast as it removes tantalum. Indeed, the present inventors have recognised a problem with electro-polishing a prosthesis such as the one disclosed in the WO document because, for any given electrolyte, the rate of electro-polishing of the tantalum spoon is liable to be substantially different from the rate of polishing of the nickel titanium prosthesis material.
Polishing the tantalum spoon separate from the nickel titanium stent matrix would be one way to manage the rate of removal of material from the surface being polished. However, electro-polishing is always liable to introduce some uncertainty as to the precise dimensions of the polished workpiece. Thus, when two components of a workpiece are to be approximated precisely, and then welded together, any electro-polishing prior to welding is liable to detract from the precision of placement of the respective components each side of the welding interface. Any such loss of precision can reduce the level of confidence in the integrity of the weld because, ideally, the gap between the components at the welding interface, to be filled by weld metal, should be precisely defined, and constant.
How, then, is one to reconcile integrity of the welded joint with precision polishing of the components of different metals? This is the problem that the present inventors addressed, and the present invention represents a solution to that problem.
In this specification, the word “polishing” covers any method of removing edges, surface roughness or imperfections. Electropolishing is one way. Chemical polishing, such as etching, is another. Mechanical polishing, such as tumbling or sand-blasting, is yet another.